Ion implanters are used to treat a workpiece with an ion beam. Ion implanters may be used, for example, to implant or dope silicon wafers with a selected ion species as desired to produce n-type or p-type extrinsic materials in fabricating semiconductor integrated circuits.
Typical ion implanters comprise an ion source for generating positively-charged ions from ionizable source materials and beam forming and shaping structure for forming the generated ions into a beam and for accelerating the ions along a predetermined path for implantation into one or more wafers at an implantation station. As the ion beam strikes the wafer, however, the wafer acquires a net positive charge. Such charging is often non-uniform and can create large electric fields at the surface of the wafer, possibly damaging the wafer. Wafers containing oxide layers, for example, can experience dielectric breakdown when wafer charging occurs.
To prevent damaging wafers in this manner, typical implanters comprise an electron shower or flood system to introduce low-energy electrons in the vicinity of the ion beam to neutralize the positively-charged ion beam and wafer surface. In a plasma electron flood (PEF) system, for example, a filament is heated to emit thermoelectrons within an arc chamber having a positive voltage bias with respect to the filament. The thermoelectrons react with a suitable gas supplied to the arc chamber to produce a plasma comprising neutralizing electrons that may be introduced in the vicinity of the ion beam through an extraction orifice in the arc chamber.
The continued trend toward thinner gate oxides requires relatively stringent control over charge neutralization to control electrical stress of gate oxide during ion implantation. Because the charge neutralization operation performed by a PEF system depends on a number of factors such as, for example, filament current, filament condition, arc current, arc chamber condition, gas purity, gas flow, proper assembly of the PEF system, etc., the operation of the PEF system should be monitored to help control charge neutralization for relatively charge-sensitive devices.